The present invention is more particularly concerned with a sensor system for providing a signal characterising the position of a vehicle driver operated command member and/or of a control member in a control system for an internal combustion engine.
In the control of an internal combustion engine in a motor vehicle, it is necessary to actuate a regulating member in the form of a butterfly valve in the intake manifold system of the engine in response to driver actuation of the accelerator pedal. Originally a mechanical linkage was used for this purpose and later a Bowden cable was preferred. Recently, advances in electronic control systems have led to the provision of a servo system for the regulating member and the first sensor responsive to the position of the accelerator pedal is in the form of a potentiometer and provides a control signal indicative of a desired position. A second sensor responsive to the actual position of the regulating member is also in the form of a potentiometer and provides a feedback signal indicative of the actual position of the regulating member. These signals are passed to a comparator which controls actuating signals to a servo motor which actuates the regulating member.
The absence of a direct mechanical connection between the accelerator pedal and the regulating member has led to the need of some form of safety device for preventing "runaway", that is, excessive engine speed such as could be caused by substantial disconformity between the actual position of the regulating member and the desired position as determined by the accelerator pedal; thus, disconformity could arise if the regulating member were in a wide open position when the accelerator pedal were in an engine idling position. For this purpose, switch contacts have been provided on the accelerator pedal and on the regulating member for controlling an engine speed reduction device. The switch contacts associated with the accelerator pedal are normally open and are closed after the pedal has been actuated a predetermined extent, while the switch contacts associated with the regulating member are normally closed and are opened when the regulating member has been moved a predetermined extent from its engine idling position. Both the switch contacts are connected in parallel to control the engine speed reduction device which is not normally operated and which when released effectively stops the engine, for example, by discontinuing the fuel supply or the ignition system. One system operating in this manner is disclosed in U.S. Pat. No. 4,305,359.
During normal engine running, the engine speed reduction device is operated. When the accelerator pedal is released and the regulating member is in the engine idling position, the circuit to the device is through the switch contacts associated with the regulating member, whereas when the accelerator pedal is depressed more than a predetermined extent, the switch contacts associated with the accelerator pedal close and provide a circuit for the device. As the regulating member is moved away from its engine idling position, at a predetermined point in its travel, the switch contacts associated with the regulating member are opened so that the circuit to the device is only through the switch contacts associated with the accelerator pedal. Ideally, one of the switch contacts would close as the other opens, but there is no guarantee that the regulating member will move in synchronism with the accelerator pedal and allowance has to be made for some time delay. Additionally, setting of the switch contacts to be actuated at precisely corresponding positions of the regulating member and accelerator pedal presents great difficulties and so, as a compromise, an overlap of closure of the switch contacts has to be arranged. A further disadvantage is that in the event of failure of the potentiometer associated with the accelerator pedal only a very restricted form of control based on the switch contacts associated therewith is possible and this control is virtually only two stage.